1. Field of the Invention
This invention relates to a composite substrate having a dielectric and an electrode, and an electroluminescent (EL) device using the same.
2. Background Art
The phenomenon that a material emits light upon application of an electric field is known as electroluminescence (EL). Devices utilizing this phenomenon are on commercial use as backlight in liquid crystal displays (LCD) and watches.
The EL devices include dispersion type devices of the structure that a dispersion of a powder phosphor in an organic material or enamel is sandwiched between electrodes, and thin-film type devices in which a thin-film phosphor sandwiched between two electrodes and two insulating thin films is formed on an electrically insulating substrate. For each type, the drive modes include DC voltage drive mode and AC voltage drive mode. The dispersion type EL devices are known from the past and have the advantage of easy manufacture, but their use is limited because of a low luminance and a short lifetime. On the other hand, the thin-film type EL devices have markedly spread the practical range of EL device application by virtue of a high luminance and a long lifetime.
In prior art thin-film type EL devices, the predominant structure is such that blue sheet glass customarily used in liquid crystal displays and plasma display panels (PDP) is employed as the substrate, a transparent electrode of ITO or the like is used as the electrode in contact with the substrate, and the phosphor emits light which exits from the substrate side. Among phosphor materials, Mn-doped ZnS which emits yellowish orange light has been often used from the standpoints of ease of deposition and light emitting characteristics. The use of phosphor materials which emit light in the primaries of red, green and blue is essential to manufacture color displays. Engineers continued research on candidate phosphor materials such as Ce-doped SrS and Tm-doped ZnS for blue light emission, Sm-doped ZnS and Eu-doped CaS for red light emission, and Tb-doped ZnS and Ce-doped CaS for green light emission. However, problems of emission luminance, luminous efficiency and color purity remain outstanding until now, and none of these materials have reached the practical level.
High-temperature film deposition and high-temperature heat treatment following deposition are known to be promising as means for solving these problems. When such a process is employed, use of blue sheet glass as the substrate is unacceptable from the standpoint of heat resistance. Quartz substrates having heat resistance are under consideration, but they are not adequate in such applications requiring a large surface area as in displays because the quartz substrates are very expensive.
It was recently reported that a device was developed using an electrically insulating ceramic substrate as the substrate and a thick-film dielectric instead of a thin-film insulator under the phosphor, as disclosed in JP-A 7-50197 and JP-B 7-44072.
FIG. 2 illustrates the basic structure of this device. The EL device in FIG. 2 is structured such that a lower electrode 12, a thick-film dielectric layer 13, a light emitting layer 14, a thin-film insulating layer 15 and an upper electrode 16 are successively formed on a substrate 11 of ceramic or similar material. Since the light emitted by the phosphor exits from the upper side of the EL structure opposite to the substrate as opposed to the prior art structure, the upper electrode is a transparent electrode.
In this device, the thick-film dielectric has a thickness of several tens of microns which is about several hundred to several thousand times the thickness of the thin-film insulator. This offers advantages including a minimized chance of breakdown caused by pinholes or the like, high reliability, and high manufacturing yields.
Use of the thick dielectric invites a drop of the voltage applied to the phosphor layer, which is overcome by using a high-permittivity material as the dielectric layer. Use of the ceramic substrate and the thick-film dielectric permits a higher temperature for heat treatment. As a result, it becomes possible to deposit a light emitting material having good luminescent characteristics, which was impossible in the prior art because of the presence of crystal defects.
Preferred conditions for the dielectric material used as the thick-film dielectric include high permittivity, insulation resistance, and dielectric strength. When the substrate material is widespread crystallized glass or Al2O3 and the dielectric material is BaTiO3 which is widely used as capacitor material because of good dielectric characteristics, there arises a problem that cracks develop in the BaTiO3 dielectric layer upon firing. Since the dielectric layer has a reduced dielectric strength due to such cracks, an EL device fabricated using this composite substrate is likely to break down. The cause is presumably the difference in coefficient of thermal expansion between the substrate material and the dielectric, which has a significant influence since the dielectric must be fired at high temperatures. Because of this problem and the need to minimize the reaction of the dielectric material with the substrate material, lead-base dielectric materials having a relatively low firing temperature have been under predominant consideration as the dielectric material, as disclosed in JP-A 7-50197 and JP-B 7-44072.
However, the use of harmful lead in the raw material is undesirable from the manufacturing standpoint and because the cost of waste recovery is increased. Still worse, lead-base dielectric materials generally have a lower firing temperature than BaTiO3, which prevents the heat treating temperature of a phosphor layer from being elevated, so that EL devices using them fail to provide satisfactory luminescent characteristics.
An object of the invention is to provide a composite substrate which suppresses reaction of a substrate with a dielectric layer that can otherwise cause degradation of the dielectric layer and which can be sintered at high temperature while minimizing the generation of cracks in the dielectric layer, as well as an EL device using the same.
The above object is attained by the following construction.
(1) A composite substrate in which an electrode and a dielectric layer are successively formed on an electrically insulating substrate,
said substrate having a coefficient of thermal expansion of 10 to 20 ppm/K.
(2) The composite substrate of (1) wherein said substrate is composed mainly of magnesia (MgO), steatite (MgO.SiO2) or forsterite (2MgO.SiO2).
(3) The composite substrate of (1) or (2) wherein said dielectric layer is a sintered ceramic body composed mainly of barium titanate (BaTiO3).
(4) The composite substrate of (3) wherein said dielectric layer contains one or more oxides selected from the group consisting of manganese oxide (MnO), magnesium oxide (MgO), tungsten oxide (WO3), calcium oxide (CaO), zirconium oxide (ZrO2), niobium oxide (Nb2O5) and cobalt oxide (Co2O3).
(5) The composite substrate of (3) or (4) wherein said dielectric layer contains the oxides of one or more elements selected from the group consisting of rare earth elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.
(6) The composite substrate of any one of (3) to (5) wherein said dielectric layer contains a vitreous component composed of silicon oxide (SiO2).
(7) An EL device comprising at least a light emitting layer and a second electrode on the composite substrate of any one of (1) to (6).
(8) The EL device of (7) further comprising a second insulator layer between the light emitting layer and the second electrode.
Since the specific substrate material and the dielectric material of the specific composition are used according to the invention, there is fabricated a composite substrate which can be sintered at a high temperature without incurring reaction of the dielectric layer with the substrate that can otherwise cause degradation of the dielectric layer and which has a thick-film dielectric layer free of cracks.
When an EL device is fabricated using the composite substrate having such a high firing temperature, the heat treating temperature of a phosphor layer can be increased whereby crystal defects in the phosphor layer are reduced and improved luminescent characteristics are obtainable. This function is effective especially when a Ce-doped SrS phosphor layer capable of emitting blue light is deposited. The dielectric layer has a high dielectric strength due to the absence of cracks, allowing high voltage drive ensuring improved luminescent characteristics.